Medulloblastoma, the most common malignant brain tumor in childhood, continues to represent a therapeutic challenge. Despite treatment with increasingly sophisticated surgical and radiotherapeutic intervention, the majority of children with this tumor will ultimately die of progressive disease. Although the role of chemotherapy in the treatment of medulloblastoma remains poorly defined, recently clinical and laboratory studies support the activity of the classical bifunctional alkylating agents against these tumors. Studies in our laboratory have documented the activity of a series of bifunctional alkylating agents including cyclophosphamide and melphalan in the treatment of a panel of medulloblastoma cell lines and transplantable xenografts. Clinical trials have confirmed the activity of cyclophosphamide and melphalan in the treatment of patients with recurrent medulloblastoma. Unfortunately, meaningful increases in the survival of patients with medulloblastoma have not yet resulted from the use of alkylating agents, a limitation due in large part to intrinsic or acquired alkylator resistance in a subset of cells in these tumors. The hypothesis of this proposal is that modulation of glutathione metabolism can enhance bifunctional alkylating agent therapy of sensitive and resistant tumor cells. The specific aims of this proposal are: 1) To define the role and mechanism(s) of glutathione in modulating the activity of melphalan and cyclophosphamide (4- hydroperoxycyclophosphamide) in alkylator-sensitive and -resistant human medulloblastoma cell lines and xenografts in athymic nude mice, 2) to define the effects of gamma-glutamylcysteine synthetase and gamma-glutamyl transpeptidase inhibitors on the activity and toxicity of nitrogen mustard- based alkylating agents in the treatment of alkylator-sensitive and - resistant human medulloblastoma cell lines and xenografts in athymic nude mice, 3) to define the effects of BSO-mediated glutathione depletion on the activity and toxicity of intrathecal malphalan in the treatment of leptomeningeal human medulloblastoma in athymic nude rats.